This invention relates to electromagnetic interference (EMI) shielding, and more particularly to a gasket for shielding such interference.
It is critical today to shield electronic equipment, such as computers, against EMI emissions. In the past, computer products worked with slower processors. Consequently, the problem of escaping emissions of these types though openings in computer enclosures was not significant. But the advent of faster processors and switching has changed this. While increased processing speeds have made remarkable improvements in computer performance and efficiency, EMI containment problems have been exacerbated.
Today it is not uncommon for computers to use clock speeds in the range of 50 to 200 MHz or more. And digital circuits operating at these high frequencies may have pulse rise times of a nanosecond or less. These speeds can cause EMI to be radiated at harmonic frequencies in excess of 1 GHz.
The Federal Communications Commission (FCC) has established standards that limit the amount of allowable EMI emissions from electronic devices. To meet FCC emissions standards it is necessary to seal around slots holding expansion or option cards and to seal around slots covered by blanks when these slots are not being used. This task is challenging in today's commercial environment.
Then too, there is commercial pressure to compact the spacing between expansion slots or openings, while still providing standard EISA/ISA expansion slots or openings. These standard slot sizes are needed to maintain interchangeability of expansion or option cards between computers even as market forces press to reduce the size of computers. This need has not been effectively met.
In the past one approach to sealing around slots and openings has been the use of individual shielding strips or clips to inhibit EMI emissions though such openings in enclosures containing electronic equipment giving off such emissions during its operations. U.S. Pat. No. 5,029,254 discloses an example of a shielding strip, which is held in place at the edge of an opening by a clamping element. During mounting of expansion or option cards, shielding strips and clips are easily knocked off. Mounting is difficult at best.
U.S. Pat. No. 4,659,869 discloses another example of a clip-on shielding strip. Shielding strips like those disclosed in these two examples present considerable difficulty. They are individually placed in position on the edges of enclosures and have the same difficultly as those mentioned in connection with the shielding strips disclosed U.S. Pat. No. 5,029,254.
Accordingly, there still exits a need for a shielding arrangement that is easily installed and that effectively retards EMI emissions.